1. Field of the Invention
This invention relates to semiconductor processing and, more particularly, to a method, system, and storage medium for determination of reticle tilt in a lithographic system.
2. Description of the Relevant Art
Fabrication of integrated circuits upon semiconductor substrates (xe2x80x9cwafersxe2x80x9d) involves numerous processing steps. For example, the fabrication of a metal-oxide-semiconductor (xe2x80x9cMOSxe2x80x9d) integrated circuit includes the formation of trench isolation structures within a semiconductor substrate to separate each MOS field-effect transistor (xe2x80x9cMOSFETxe2x80x9d) that will be made. The semiconductor substrate is typically doped with either n-type or p-type impurities. A gate dielectric, typically composed of silicon dioxide, is formed on the semiconductor substrate. For each MOSFET being made, a gate conductor is formed over the gate dielectric and a source and drain are formed by introducing dopant impurities into the semiconductor substrate. Conductive interconnect lines are then formed to connect the MOSFETs to each other and to the terminals of the completed integrated circuit. Modem high-density integrated circuits typically include multiple interconnect levels to provide all of the necessary connections. Multiple interconnect levels are stacked on top of each other with intervening dielectric levels providing electrical insulation between interconnect levels.
During integrated circuit fabrication, various structures, including the trench isolation structure, gate conductor, and interconnect lines, of the circuit are patterned, typically using optical lithography. Lithography is process whereby a pattern is transferred from a mask onto the semiconductor substrate. Lithography begins by applying a photosensitive material, often called photoresist, as a thin film to an upper surface of the wafer. Light is then projected onto the photoresist through the mask, which contains clear and opaque features that define a pattern to be created in the photoresist. The portions of the photoresist exposed to the light are rendered either soluble or insoluble in a specific solvent, often called developer. After the developer removes soluble portions of the photoresist, the remaining photoresist forms a mask that may be used in patterning layers of the wafer underneath the photoresist. For example, the photoresist may be used to protect the covered portions of the semiconductor substrate while the exposed portions of the semiconductor substrate are etched.
One commonly used lithographic system is the step and repeat projection system in which the mask, referred to as a reticle, can pattern only a portion of the photoresist. The step and repeat system projects a pattern of the reticle onto a portion of the semiconductor substrate. The system then moves the semiconductor substrate and projects the pattern of the reticle onto a different portion of the semiconductor substrate. This process is repeated until the reticle""s pattern has been projected onto the entirety of the semiconductor substrate. Since multiple, identical integrated circuits are manufactured on the large diameter (typical diameterxe2x80x94200 mm) semiconductor substrates used in modern integrated circuit fabrication, the reticle will pattern a portion of the photoresist corresponding to one or more circuits. For example, the reticle may pattern four seperate die simultaneously. Step and repeat projection system also often have lenses placed between the reticle and the semiconductor substrate such that the pattern projected onto the photoresist is smaller than the pattern of the reticle. Pattern reduction of a factor of five is common.
A prevalent trend in modern integrated circuit fabrication is reducing the size of circuit structures, such as gate conductors and interconnect lines, to permit circuits of greater complexity to be manufactured without substantially increasing the area occupied by the circuit. This continuing reduction in size places ever greater demands on the lithography system to increase its resolution. Rayleighs criterion defines the minimum distance between two features in the pattern that are resolvable: Two separate features are resolvable if 2d=0.61 xcex/NA where 2d is the distance between the features, xcex is the wavelength of the of light projected by the lithographic system, and NA is the numerical aperture of the lens that focuses light from the reticle onto the photoresist. Resolution is typically increased (i.e., reducing 2d) by either decreasing the wavelength of light used or by increasing the numerical aperture of the lens.
Depth of focus is another critical property of the lithographic system. Depth of focus refers to the ability to focus light over only a limited focal plane. Depth of focus "sgr" is given by "sgr"=xcex/(NA)2 where xcex is the wavelength of the of light and NA is the numerical aperture of the lens as before. Improvements in the resolution of the lithographic system by decreasing the wavelength or increasing the numerical aperture unfortunately results in a decrease of the depth of focus. Modern step and repeat projection system, which typically use light with wavelengths of 365 nm (I-line) or 248 nm (deep UV) and have numerical apertures greater than 0.6, often have a depth of focus of less than 1 xcexcm.
If any portion of the photoresist is outside of the focal plane of the lithographic system, that portion of the photoresist will be improperly exposed resulting in incomplete transfer of the reticle pattern onto the photoresist. The developed photoresist will therefore not have the desired pattern and the photoresist may not protect the proper portions of the underlying semiconductor substrate when the photoresist is used as a mask. For example, if the photoresist is being used to mask a metal layer that is being etched to form interconnects, a defective mask may result in two interconnects being shorted or may result in an interconnect having a width less than that targeted and therefore a current carrying capacity less than that targeted. Both situations may lead to failure of the integrated circuit being manufactured. In general, underexposure of the photoresist will decrease the yield of the fabricated integrated circuits and increase the costs of manufacturing. The yield is defined as the percentage of completed integrated circuits manufactured that are functional.
The limited depth of focus of modem lithographic systems requires extremely tight tolerances on the positioning of all components in the system including the reticle and semiconductor substrate. For example, both the upper surface of the semiconductor substrate and the photoresist layer must be planar. Also, both the reticle and semiconductor must be positioned the correct distances from one another. Additionally, if the reticle is tilted, portions of the photoresist may be outside of the focal plane and those portions may therefore be underexposed. Having one side of the reticle out of position by as little as 0.3 xcexcm may result in an unacceptable amount of reticle tilt. In general, modem lithographic systems have the ability to precisely adjust the position and tilt of the reticle.
To aid in verifying proper performance of the lithographic system, test patterns are typically included on reticles in addition to patterns corresponding to the circuit being manufactured. The test patterns are arranged on the reticle such that the test patterns formed in the photoresist are in areas of the semiconductor substrate external to areas of the semiconductor substrate in which circuits are being fabricated. Examination of the test patterns on the semiconductor substrate after development of the photoresist can yield information about the performance of the lithographic system.
An unacceptable amount of reticle tilt is generally checked for by examining multiple test patterns on the semiconductor substrate. Due to their small size, test patterns are typically imaged using a scanning electron microscope. If every test pattern has the correct shape, the reticle is considered to positioned correctly; whereas, if one or more of the test patterns do not have the correct shape, the reticle is considered to have an unacceptable amount of reticle tilt. The shape of the test pattern refers to the three dimensional structure of the test pattern. The imaging and assessment of the test patterns is generally done manually by an operator. If an unacceptable reticle tilt is found, the position of the reticle in the lithographic system must be adjusted.
Since the manual process of imaging and assessing the test patterns is relatively slow and time consuming, it is typically done only on representative semiconductor substrates. Often reticle tilt may be checked only once a day. If the reticle tilt drifts over the course of the day, many semiconductor substrate can be processed by the lithographic system before the reticle tilt is discovered. These improperly processed semiconductor substrates may exhibit decreased yield and therefore result in increased costs of manufacturing. Additionally, the yield may not be decrease but the appearance of the completed circuit may be unappealing to the customer who purchases the circuit.
Additionally, since the assessment of the test patterns is performed manually, the assessment may include operator bias. For instance, different operators may assess similar test patterns differently. Additionally, the same operator assessment of similar test patterns may change over time. In either case, the possibility exists that in some cases unacceptable reticle tilt may go unnoticed resulting in decreased yield and increased manufacturing costs.
It is therefore desirable to develop an improved method for quickly determining if an unacceptable amount of reticle tilt is present. A quicker determination of reticle tilt would allow the reticle tilt to be checked more often thereby minimizing the quantity of semiconductor substrates processed by the lithographic system with an unacceptable amount of reticle tilt. It is also desirable to remove operator bias from the determination of reticle tilt so that similar test patterns always result in the same assessment of reticle tilt and unacceptable reticle tilt will not go unnoticed.
The problems outlined above are in large part addressed by a method in which images of test patterns located on a semiconductor substrate are automatically measured by a scanning electron microscope, transferred to a computer system, and are assessed by the computer system to determine if there is an unacceptable amount of reticle tilt. Automatic measurement of test pattern images by the scanning electron microscope and assessment of the measured images by the computer system advantageously increases the speed at which detection of reticle tilt may be accomplished. Since automatic assessment of test patterns can be accomplished quicker, the frequency with which the test patterns are assessed may also be increased. More frequent assessment of test patterns for the presence of unacceptable reticle tilt may reduce the quantity of semiconductor substrates processed by the lithographic system before the unacceptable reticle tilt is discovered. Additionally, utilization of a computer system to assess the images of the test patterns removes any operator bias from the process and assures that similar test patterns will always be assessed identically.
The measurement of the images by the scanning electron microscope is preferably controlled by the computer system. The computer-controlled process includes loading the semiconductor substrate, on which the test patterns are located, into the scanning electron microscope. The position of the semiconductor substrate is then adjusted such that the approximate location of one of the test patterns is within a field of view of the scanning electron microscope. The test pattern is then located and centered within the field of view of the scanning electron microscope. Finally, an image of the test pattern is sent to the computer system. The process may be repeated for each of the remaining test patterns.
Once the computer system has received the images of the test patterns, the computer system may assess the images to determine whether the reticle tilt is acceptable or unacceptable. In one embodiment, the computer system may compare the measured images of each test pattern to predetermined images of the test pattern for different focus conditions. Different focus conditions are the result of different positions of the reticle within the lithographic system. Changing the position of the reticle will change the position at which an image of the reticle""s pattern will form. Each of the different focus conditions are predetermined to be either acceptable or unacceptable. The computer system finds the predetermined image that best matches each of the measured images. If the computer system finds that each measured image of the test patterns best matches with a predetermined image that corresponds to acceptable focus conditions, the computer system determines that the reticle tilt is acceptable. However, if the computer system finds that at least one of the measured images of the test patterns best matches with a predetermined image that corresponds to unacceptable focus conditions, the computer system determines that the reticle tilt is unacceptable. If the reticle tilt is determined to be unacceptable, the computer system may then notify an operator of the lithographic system and/or may signal a computer, which controls the lithographic system, to take the lithographic system offline, so that correction of the reticle tilt may be accomplished.
In an additional embodiment, the computer system may also calculate the amount of reticle tilt. The computer system may inform an operator of the calculated amount of reticle tilt. The operator may then adjust the position of the reticle within the lithographic system such that the tilt is removed. The computer system may calculate the amount of reticle tilt based upon the focus conditions, which correspond to a specific reticle position, of the predetermined images that best matches each of the measured images.
In an alternative embodiment, the computer system may compare a first image of the measured images to the other measured images to determine whether the reticle tilt is acceptable or unacceptable. If the reticle is not tilted, all the measured images should be essentially identical. The greater the amount of reticle tilt, the more dissimilar the measured images will be. The computer system determines a degree to which the first measured image matches the other measured images. If the first measured image matches the other measured images to within a predetermined amount, the computer system determines the reticle tilt to be acceptable. If the first measured image does not match at least one of the other measured images to within the predetermined amount, the computer system determines the reticle tilt to be unacceptable. As with the previous embodiment, if the reticle tilt is determined to be unacceptable, the computer system may then notify an operator of the lithographic system and/or may signal a computer, which controls the lithographic system, to take the lithographic system offline, so that correction of the reticle tilt may be accomplished.
In addition to the method described above, a system is contemplated herein. The system comprises a computer system connected to a scanning electron microscope. A database on the computer system maintains a record of measured and predetermined images of test patterns. A program executing on the computer system can control the scanning electron microscope such that images of test patterns on a semiconductor substrate are measured. In one embodiment, the program may then compare the measured images and the predetermined images to determine the reticle tilt to be acceptable or unacceptable. The program may also calculate the amount of reticle tilt. In another embodiment, the program may compare the measured images to each other to determine the reticle tilt to be acceptable or unacceptable. Additionally, the program may display the measured images of the test patterns on an output device, such as a monitor or printer, so that an operator may independently assess the measured images.
A computer-readable storage medium is also contemplated herein. The storage medium contains program instructions that can be implemented by an executable unit to control a scanning electron microscope and assess measured images of test patterns. The storage medium includes measured and predetermined test pattern image data. The measured test pattern image data corresponds to images of test patterns on a semiconductor substrate measured by the scanning electron microscope. The predetermined test pattern image data corresponds to predetermined images of the test pattern for different focus conditions. In one embodiment, the instructions cause the executable unit to compare the measured images to the predetermined images and to determine if the reticle tilt is acceptable or unacceptable. The instruction may also cause the executable unit to calculate the amount of reticle tilt. In an additional embodiment, the instructions cause the executable unit to compare the measured images to each of the predetermined images to determine if the reticle tilt is acceptable or unacceptable. In another embodiment, the instructions cause the executable unit to compare the measured images to each other to determine if the reticle tilt is acceptable or unacceptable.